1. Field of the Invention
The present invention generally relates to a method for forming a gate insulating film. More particularly, the present invention relates to a method for forming a gate insulating film made of a high dielectric constant metal silicate or a high dielectric constant metal oxide.
2. Related Art
With recent improvement in integration degree, functions, and operation speed of semiconductor integrated circuit devices, dimensions of semiconductor elements such as transistors have been reduced. Therefore, not only lateral dimensions of semiconductor elements (dimensions in an in-plane direction of a substrate) such as the gate length of transistors but also longitudinal dimensions thereof such as the thickness of a gate insulating film have been reduced. A silicon oxide film (SiO2 film) has been widely used as a gate insulating film due to its affinity to a semiconductor substrate made of silicon and its insulation performance. However, the relative dielectric constant of SiO2 is only 3.9, and significant reduction in physical film thickness has been demanded in order to increase the capacitance. An ultrathin SiO2 film of less than 2 nm thickness has therefore been used in recent years. In the case where a SiO2 film of less than 2 nm thickness is used, however, a direct tunneling current flows instead of a Fowler-Nordheim tunneling current. This increases a leakage current, making it difficult to suppress power consumption not only during operation of a device but also in a stand-by state.
A silicon oxynitride film (SiON film) has therefore been increasingly used to avoid this problem. The silicon oxynitride film is a SiO2 film having nitrogen introduced therein in order to increase the relative dielectric constant to about 5, that is, an intermediate value between the relative dielectric constant (7) of a Si3N4 film and the relative dielectric constant (3.9) of a SiO2 film. However, the relative dielectric constant value increases by only 25% and the leakage current decreases by only one digit or less. It is therefore extremely difficult to implement an EOT (Equivalent Oxide Thickness) of 1.5 nm or less even when a SiON film is used as a gate insulating film.
It has been considered to use a high dielectric constant metal oxide film or a high dielectric constant metal silicate film having a high relative dielectric constant as a gate insulating film. Especially, a hafnium oxide (HfOx) film or a hafnium silicate (HfSiO) film has drawn attention because a relative dielectric constant of 25 or 14 to 18 is obtained.
FIG. 12A shows a relation between the EOT and the leakage current of a SiO2 film, a SiON film, and so-called High-K insulating films such as a high dielectric constant metal oxide film and a high dielectric constant metal silicate film. It can be seen from FIG. 12A that the High-K insulating films capable of suppressing the leakage current by several digits as compared to the SiO2 film and the SiON film are promising as a next-generation ultrathin gate insulating film. However, further reduction in leakage current has been demanded for the High-K films as well. A HfO, film and a HfSiO film can be deposited by a known MOCVD (Metal-Organic Chemical Vapor Deposition) method. However, since a thin thickness of 1 nm to 2 nm has been demanded for such a high dielectric constant metal oxide film and a high dielectric constant metal silicate film as well, it has becoming common to use an ALD (Atomic Layer Deposition) method in addition to a conventionally used MOCVD method. FIG. 12B shows a gate structure of a conventional transistor having a high dielectric constant metal silicate film as a gate insulating film. In this transistor, an interface layer 102, a gate insulating film 103, and a gate electrode 104 are sequentially formed on a semiconductor substrate 101. For example, the semiconductor substrate 101 is made of silicon, the interface layer 102 is made of SiO2 or the like, the gate insulating film 103 is made of a high dielectric constant metal silicate, and the gate electrode 104 are made of a polycrystalline silicon, a metal, or the like.
As shown in FIG. 13A, in a conventional ALD method, a metal film corresponding to one layer is formed by performing the following steps as a cycle: the steps of exposing a semiconductor substrate to a precursor (Hf) of a metal of a high dielectric constant metal oxide film for a predetermined time, purging the precursor, exposing the semiconductor substrate to an oxidizing agent (O3) for a predetermined time, and purging the oxidizing agent. A desired film thickness is obtained by repeating this cycle a plurality of times.
A so-called Nano-laminate method is used when a gate insulating film is made of multiternary compounds, for example, when a gate insulating film is made of a high dielectric constant metal silicate film. In this method, after a high dielectric constant metal oxide film is deposited in a predetermined cycle, a silicon oxide film corresponding to one layer is formed by performing the following steps as a cycle: the steps of exposing a substrate to a precursor (Si) for a predetermined time, purging the precursor, exposing the substrate to an oxidizing agent for a predetermined time, and purging the oxidizing agent. A high dielectric constant metal oxide film and a silicon oxide film are thus alternately deposited, and this alternate formation of the high dielectric constant metal oxide film and the silicon oxide film is repeated until a desired film thickness is obtained.
In a high dielectric constant metal silicate of a gate insulating film, the composition ratio of a high dielectric constant metal to silicon is important because the composition ratio determines the relative dielectric constant, EOT, and leakage current of the high dielectric constant metal silicate. The composition ratio is determined by adjusting the number of deposition cycles of the high dielectric constant metal oxide film and the number of deposition cycles of the silicon oxide film. For example, in a high dielectric constant metal oxide film, as shown in FIG. 13B, the composition ratio of Hf to Si becomes 1:1 (Hf=50%) when a cycle of a HfOx film and a cycle of a SiO2 film are alternately repeated a plurality of times. As shown in FIG. 13B, the composition ratio of Hf to Si becomes 2:1 (Hf=66.7%) when two cycles of a HfOx film and a cycle of a SiO2 film are alternately repeated a plurality of times. In this way, the composition of the high dielectric constant metal silicate can be adjusted. The composition ratio of a metal to Si is thus strictly controlled in the ALD method.
It has been pointed out that the ALD method has poor productivity because a metal layer and an oxygen layer are formed on a layer-by-layer basis on an atomic layer level. It has therefore been demanded to minimize the exposure time to a precursor of each material, the purge time of the precursor, and the exposure time to an oxidizing agent. It has been considered that, in the ALD method, deposition of an atomic layer is caused by adsorption of atoms to the surface of a processed film. The ALD method is based on a mechanism in which deposition stops when atoms corresponding to one layer have been completely adsorbed by the surface of a processed film. Accordingly, when the exposure time to a precursor and the exposure time to an oxidizing agent are varied, these exposure times are determined by the saturation time of the film thickness of one layer to be deposited in a cycle.
Of course, the minimum saturation time of the thickness of a deposited film is selected, and the exposure time to a precursor and the exposure time to an oxidizing agent are set so as to maximize the film thickness of a layer to be deposited per cycle. These exposure times therefore tend to be short. For example, Japanese National Phase PCT Laid-Open Patent Publication No. 2007-519225 discloses that the exposure time to a precursor is 0.2 seconds to 0.5 seconds and the exposure time to an oxidizing agent is 2 seconds in an ALD cycle.
Japanese National Phase PCT Laid-Open Patent Publication No. 2004-511909 discloses that the exposure time to a precursor is 1.5 seconds and the exposure time to an oxidizing agent is 3 seconds. The exposure time to a precursor is set to various values depending on the type of the precursor, whereas the exposure time to an oxidizing agent is set approximately to 1 second to 3 seconds.